Pneumatic tires are commonly used in on-road vehicles such as automobiles and trucks. Pneumatic tires have the advantages of being light in weight and providing a soft and comfortable ride, because the tire casing is filled with a gas or air. The main disadvantage of pneumatic tires is the risk of deflation due to punctures, separation of the tire casing from the rim, or other failure of the tire casing or rim.
U.S. Pat. No. 6,303,060 discloses a non-deflatable tire assembly having a hollow toroid-shaped flanged insert and a flexible body molded around the insert to form a tire, and at least one rim onto which is mounted the tire. The flexible body is made of a polyurethane foam formed of a combination of polyol and polyisocyanate, while the insert is made of another material such as high density polyethylene. The hollow insert itself is required to provide load carrying stiffness to the tire, thereby contributing to a cushioned ride. Although the hollow internal area is reduced in dimension, the tire is not completely non-deflatable and hollow areas are still required to achieve acceptable mechanical properties.
Furthermore, the polyurethane foam used has a too high compression set value (in the range 10-20%).
U.S. Pat. No. 4,125,691 discloses a zero pressure device composed of either a microcellular or homogeneous polyurethane made by reacting an organic polyisocyanate with at least three polyols, a monomeric polyol of 2 to 3 hydroxyls having a molecular weight less than 250, a polyether triol having a molecular weight of 4600 to 6000 and a polyether glycol having a molecular weight of 3500 to 4200. The organic polyisocyanate used is a quasi prepolymer made by reacting MDI with a polypropylene ether triol. US′691 further discloses examples wherein 2 pbw acrylonitrile grafted polypropylene ether glycol (polymer polyol) is added to the reactive mixture. Using polypropylene based quasi prepolymers will lead to polyurethane materials having a too low ball rebound and a too low hardness will be achieved due to insufficient amounts of polymer polyols.
For above reasons, filled tires are more attractive than pneumatic tires. A filled tire contains a solid or semi-solid material instead of a compressed gas. This eliminates the risk of deflation, as a puncture or other failure of the tire casing will not lead to an escape of gas.
However a tire fill material should meet several requirements to compete with the good dynamic properties of pneumatic tires. For example the tire fill material should allow the tire to absorb shock and provide good traction. Therefore, the tire fill material should be soft and flexible. In addition, the tire fill material should be such that the tire does not build up excessive heat during use, as the heat can damage the fill material or the casing and thus diminish the useful life of the tire. In addition, cost is a very important concern.
Soft polyurethane/urea elastomers have been used as a tire fill material in the past and several approaches have been tried. In some cases, the polyurethane/urea polymer has been foamed using carbon dioxide that is generated in a reaction between water and an organic isocyanate. Such an approach is described in U.S. Pat. No. 3,605,848. These foams have the advantages of light weight due to their cellular nature, and of being too soft and the foams tend to exhibit high hysteresis and high heat build-up.
Polyesterol-containing soft phases lead to the highest level of dynamic properties in cellular PU elastomers having a urea hard phase. Products of this type are also well known. For example WO 2001018086 describes the use of polyester polyetherol obtainable via polycondensation of polyoxytetramethylene glycol whose average molar mass is from 220 to 270 g/mol and adipic acid, for the production of cellular polyurethane elastomers with good dynamic properties and with high low-temperature flexibility. However, the ester bonds make the resultant foam susceptible to hydrolysis.
DE-A 3613964 describes the production of products based on pure polyester soft phases and, respectively, polyester-polyetherol soft phases. The test specimens based on polytetrahydrofuran (M=2000 g/mol) as soft phase in the comparative example in DE-A 3613964 had only comparatively low flexural strength.
The overall disadvantage of the cellular polyurethane elastomers known in the prior art and used for tires is fact that they do not retain the desired shape if the polyurethane elastomer is subject to an increased temperature and they do not withstand contact with moisture and/or to high pressure acting on the elastomer.
For all reasons above indicated there is a need to develop a new polyurethane elastomeric material which has very good dynamic properties and which is able do retain the desired shape if the polyurethane elastomer is subject to an increased temperature and to withstand contact with moisture and/or to high pressure acting on the elastomer.